1) Field of the Invention
The present invention relates to a method for starting continuous casting in continuous slab casting and a method for setting a dummy bar prior to the start of casting of continuous slab casting.
2) Prior Art
When continuous slab casting is started in a conventional width-variable continuous casting apparatus by setting a dummy bar in a mold at the initial starting period of width-variable continuous slab casting apparatus, the following method has been so far employed.
FIG. 3(a) is a plan view schematically showing a width-variable mold used in the conventional continuous slab casting, and FIG. 3(b) is a vertical cross-sectional view along the line A--A of FIG. 3(a), where numeral 1 is longitudinal side plates of a mold 10, and 2 is lateral side plates of the mold. The lateral side plates 2 are moved in the width direction of a slab by pairs of upper and lower driving units 3a and 3b provided at the outsides of the lateral side plates 2 in such a manner that the lateral side plates 2 are tilted, while being pinched by the longitudinal side plates 1, and thereby to change or adjust the slab width dimension. In that case, the distance between the longitudinal side plates 1 is usually kept constant. When the slab thickness is to be changed, another procedure, for example, replacement of the lateral side plates 2 themselves, will be taken.
A procedure for starting the conventional continuous slab casting operations with the above-mentioned mold 10 will be explained below, referring to FIG. 3 and FIGS. 4(a) to 4(d).
In FIG. 4(a), when a dummy bar 4 is inserted from the upside or the downside of the mold 10, in order to facilitate its insertion, at first the driving units 3a and 3b are actuated to set positions of the top end sides of the lateral side plates 2 to a distance consisting of a desired distance (cast slab width+allowance for thermal shrinkage) and a distance for latitude, and thereby sufficiently broadening the distance between the lateral side plates 2, and then actuated to set the lateral side plates 2 to the desired distance. Or there is a case that from the beginning, the lateral side plates 2 are set to the desired distance and then the dummy bar 4 having a width 20-60 mm smaller than the distance between the lateral side plates 2 is inserted into the mold 10.
The inserted dummy bar 4 is set to a desired level at the center in the vertical direction in the mold 10, usually at a level 1/3-1/2 of the total depth of the mold 10 distant from the bottom of the mold 10, and at the same time the bottom distance between the lateral side plates 2 is set to the desired width as the bottom, giving a downwardly tapered profile to the lateral side plates 2, as shown in FIG. 4(b).
The reasons why the lateral side plates 2 are set to take the downwardly tapered profile are that since a solidified shell formed in the mold undergoes a large shrinkage particularly in the width direction of a slab as cooling proceeds, clearances are formed between the lateral side plates 2 and the solidified shell 6a, and thus in view of slab heat shrinkage which may usually occur in the stationary casting state, the downwardly tapered profile is given to the lateral side plates 2, corresponding to the slab heat shrinkage, and thereby to narrow the distance at the bottom ends thereof.
On the other hand, usually no such downwardly tapered profile as given to the lateral side plates 2 is given to the longitudinal side plates 1, because the thickness of a cast slab is several fractions smaller than the width thereof and the slab heat shrinkage is thus smaller than that of the lateral side plates 2. It is also not necessary even in the insertion of a dummy bar 4 to give an allowance to the thickness, and the clearances between the dummy bar 4 and the longitudinal side plates 1 are kept to about 2-about 5 mm, which are indispensable for the working of the dummy bar 4.
At first, the dummy bar 4 is set to the inside of the mold 10 in this manner, and then heat-resistant sealing materials 5 having a good elasticity are filled into the clearances between the mold 10 and the dummy bar 4 to prevent leakage of molten steel therethrough.
Then, molten steel 6 is poured into the mold 10 from a tundish, as shown in FIG. 4(c). The poured molten steel is held in the mold 10 for about 40 seconds so that it can be withdrawn by the dummy bar 4, and cooled by the mold 10 during that time to form a solidified shell 6a.
A recess 4a is formed on the top surface of the dummy bar 4, and the molten steel is passed into the recess 4a and solidified therein to intensify the bondage between the dummy bar 4 and the slab, thereby making it possible to withdraw the slab by the dummy bar.
The molten steel 6 is continuously poured, while withdrawing the dummy bar 4 downwardly, as shown in FIG. 4(d), and the solidified shell 6a grows at the same time during the withdrawal to form a continuous cast slab.
In the unstationary state at the start of casting, the top of the dummy bar 4, the circumference of which has been filled with sealing materials 5, and the solidified shell 6a at the initial period of starting have a larger width than that of the bottom end of the mold 10, and thus a very large pulling force is required for passage of the dummy bar 4 through the bottom end of the mold 10 due to sliding of the dummy bar 4 or the solidified shell 6a on the mold 10. When the solidified shell 6a does not throughly grow in that case, the solidified shell 6a will be broken due to the friction on the mold 10, whereby the so-called break-out occurs and the bottom end of the mold 10 is damaged. As a result, such problems as a decrease in working time, a decrease in mold life, an increase in maintenance cost, etc. appear.
To solve these problems and obtain a strength of the solidified shell 6a large enough to withstand the large pulling force, it has been proposed to provide a sufficient time of holding the cast steel in the mold 10 or provide cooling materials 11 on, for example, the top surface of the dummy bar 4 in the mold 10, as shown in FIG. 4(b) to promote the formation and solidification of the shell 6a. However, these procedures have such new problems as an increase in preparatory time for the casting and operating cost and also making it hard to automate the casting operations. Furthermore, the lateral side plates 2 are in a profile of upwardly increasing distance in that case, and thus the total of the clearances between the top end of the dummy bar 4 and the wall of the mold 10 at both sides of the dummy bar 4 are as large as 20-45 mm, resulting in difficulty of filling the sealing materials 5. This means that there are still such problems as an increase in preparatory time for the casting and making it difficult to automate the casting operations.